1. Field of the Invention
The present invention relates to a magnetic shielding case for protecting a photomultiplier from being influenced by magnetic fieldsin order to stabilize the output of the photomultiplier, and a light detecting apparatus which includes the magnetic shielding case and a photomultiplier housed within the magnetic shielding case.
2. Related Background Art
A conventional magnetic shielding case has, in general, the configuration shown in FIG. 1. This magnetic shielding case 100 has a magnetic shielding main body 101 which is cylindrically formed of permalloy having a high permeability. Further, the magnetic shielding main body 101 is provided with a rectangular entrance window 105, which faces a reflection type photocathode 104 disposed within a sealed glass envelope 103 of a side-on type photomultiplier 102. Accordingly, incident light (light to be detected) transmitted through the entrance window 105 of the magnetic shielding case 100 impinges on the photocathode 104, and photoelectrons emitted from the photocathode 104 are multiplied by an electron multiplying section 106 so as to be collected as an output signal at an anode 107.
In general, a photomultiplier of a type in which the distance between the photocathode 104 and a dynode 106a in the first stage is long are likely to be influenced by a magnetic field, whereby photoelectrons may deviate from their normal orbits under the influence of the magnetic field, thus lowering the gain. Accordingly, in order to keep the photomultiplier 102 from being influenced by external magnetic fields, the above-mentioned magnetic shielding case 100 has been utilized.